SYSTEM and Public

In a mock test, given answer to following question is a and b. I will be obliged if any one can explain this answer to me.

2. Which of the following code is valid? a) <!DOCTYPE SYSTEM SYSTEM "test.dtd"> <SYSTEM></SYSTEM> b) <!DOCTYPE PUBLIC SYSTEM "test.dtd"> <PUBLIC></PUBLIC> c) <!DOCTYPE PUBLIC PUBLIC "test.dtd"> <PUBLIC></PUBLIC> d) <!DOCTYPE SYSTEM PUBLIC "test.dtd"> <PUBLIC></PUBLIC>

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